The son of attorney and tennis player Joseph Sill Clark Sr., Clark pursued a legal career in Philadelphia after graduating from the University of Pennsylvania Law School. He became involved in a reform movement that sought to break the power of the city's Republicanpolitical machine. After serving in the United States Army Air Forces during World War II, Clark won election as city controller in 1949. In this capacity, he investigated and publicized scandals in the city government. In 1951, Clark won election as Mayor of Philadelphia, becoming the first Democrat to do so since 1884. As mayor, he sought to reduce corruption in city government and created low-income housing projects.

After one term as mayor, Clark narrowly defeated incumbent Republican Senator James H. Duff in the 1956 Senate election. Clark earned a reputation as a strong supporter of civil rights and worked to appoint liberal committee members from his perch on the Democratic Steering Committee. Clark narrowly won re-election in 1962 but was defeated in 1968 by Congressman Richard Schweiker. His defeat is generally credited to his support of gun control and opposition to the Vietnam War. After leaving office, Clark became a professor at Temple University.

In December 1926, Clark was admitted to the bar and became an associate with his father's law firm of Clark, Clark, McCarthy and Wagner.[6] That same year, he made his first entry into politics when he unsuccessfully ran as a progressive candidate for Republicancommitteeman.[8] In 1928, he left the Republican Party and became a Democrat, supporting Al Smith in that year's presidential election.[2]

Following his return to Philadelphia, Clark resumed his political activity and his partnership with Dilworth. He was manager of Dilworth's unsuccessful campaign for Mayor of Philadelphia against Republican incumbent Bernard Samuel in 1947.[2] He then served as chairman of the citizens' committee for President Harry S. Truman in the 1948 election, and as chairman of the Philadelphia chapter of the Americans for Democratic Action from 1948 to 1949.[6] Running as a reform Democrat, Clark was elected city controller in 1949, winning by more than 100,000 votes.[7] Meanwhile, Dilworth was elected city treasurer by a similar margin.[8] Serving from 1950 to 1952, Clark investigated and publicized scandals within the Republican-controlled city government, including the embezzlement of tax money and court funds, imprisonment of the fire marshal, falsification of records, and corruption in the water bureau.[6] Many officials were impeached or indicted as a result, and nine even committed suicide.[8]

Clark announced his candidacy for Mayor of Philadelphia in May 1951.[2] Democratic leaders had wanted Dilworth to run for mayor, but Clark released a press statement declaring his "irrevocable decision to run for mayor."[10] Dilworth instead successfully ran for District Attorney of Philadelphia.[10] Running on his record as city controller, Clark often used a broom while campaigning as a symbol of his pledge to "sweep out" corruption.[5] His Republican challenger was Daniel A. Poling, a Baptist clergyman and editor of the Christian Herald.[4] Clark was endorsed by several labor unions, Americans for Democratic Action, and The Philadelphia Inquirer.[6] On November 6, 1951, he defeated Poling by a margin of 124,700 votes.[8] With his victory, Clark became the first Democrat to be elected Mayor of Philadelphia since 1884.[5] As of 2015 a Democrat has held the office ever since. Clark was the last Mayor of Philadelphia to serve only one term.

Clark was inaugurated as the 90th Mayor of Philadelphia on January 7, 1952.[6] He was the first mayor to serve under Philadelphia's Home Rule Charter, which had reorganized city government by merging Philadelphia's city and county offices, establishing a limit of two successive terms for mayor, replacing patronage with a merit system for civil servants, and giving the mayor increased administrative, legislative, and investigative powers.[10] During his administration, he reduced corruption within the Police Department and appointed several African Americans to city jobs.[2] He adopted a $20 million tax increase and established a pay-as-you-go system.[8] He created low-income housing projects, also establishing the position of housing coordinator.[11] He also refused to accept personal gifts.[1] His tenure also saw the transformation of the Penn Center and the Philadelphia waterfront.[10]

In 1952, Clark launched a television series Tell It To the Mayor in which he and other city officials answered questions about his administration.[6] He endorsed Senator Estes Kefauver for the Democratic nomination in the 1952 presidential election.[6] In 1955, he chartered the Food Distribution Center Corporation to create a new food market, and established the Urban Traffic and Transportation Board to design a mass transit system.[11]

In 1956, Clark became the first politician to receive the Philadelphia Award for promoting good governance in the city.[12]

Clark, who had promised to serve as mayor for only one term, announced his candidacy for the United States Senate in 1956.[4] After winning the Democratic nomination over the opposition of Philadelphia's party leaders, he faced first-term Republican incumbent James H. Duff, a popular former governor, in the general election.[13] During the campaign, Clark ran on a liberal platform which included support for increasing the minimum wage, expanding Social Security, and repealing the Taft–Hartley Act.[14] He also criticized President Dwight D. Eisenhower on international and domestic matters, and attacked Senator Duff's poor attendance record.[14] On November 6, 1956, Clark narrowly defeated Duff by a margin of 50.1%-49.7%, winning by less than 18,000 out of 4.5 million votes cast.[15] At the same time in the presidential election, President Eisenhower, who by this time claimed his farm in Gettysburg as his permanent address, carried Pennsylvania by well over 600,000 votes.[14]

Following his departure from the Senate, Clark served as a professor at Temple University in 1969.[7] He was president of World Federalists U.S.A., an organization promoting the creation of a world government, from 1969 to 1971.[7] He also served as chairman of the Coalition on National Priorities and Military Policy, and continued to attend meetings of Members of Congress for Peace Through Law.[11] A strong opponent of Mayor Frank Rizzo, he supported Bill Green III in the 1971 Democratic primary and then Republican W. Thacher Longstreth in the general election.[2] He was chairman of independent candidate Charles Bowser's campaign in 1975.[2]

Clark died at his home in Chestnut Hill, at age 88.[5] His remains were cremated.[7]

Clark's paternal grandmother was the daughter of Joseph and Jane (née Todhunter) Sill, who were social reformers and leaders in antebellum Philadelphia's benevolence movement.[20] Joseph Sill served as secretary, vice president, and president of the St. George Society of Philadelphia, an aid organization for English immigrants.[20]

Clark was married three times and had two children. He and his first wife, Elizabeth Story Jenks, had one son, Joseph S. Clark III.[4] He was married to his second wife, Noel Hall, from April 1935 until their divorce in September 1967.[21] He and Noel had one daughter, Noel Clairborne Clark.[6] Two weeks after his divorce, Clark married Iris Cole Richey, a former editor of the Pennsylvania Manual, to whom he remained married until his death.[22]